1. Field of the Invention
The present invention is a DC offset correction circuit for use in an electronic organ. The correction circuit eliminates the audible noise or thump in the audio output signal caused by an instantaneous DC level shift in the keyer output signal at each key depression and release.
While the present invention is described herein with reference to particular embodiments, it should be understood that the invention is not limited thereto. The correction circuit of the present invention may be employed in a variety of forms, as one skilled in the art will recognize in light of the present disclosure.
2. Prior Art
The undesirable audible noise or thump in the audio signal caused by the instantaneous DC level shift in the keyer output waveform at key depression and release is commonly eliminated in electronic organs by providing a RC time constant circuit at the signal input to each keyer circuit. The RC time constant circuit provides for a very slow response by the keyer circuit so that the DC shift in the keyer output waveform is not instantaneous but rather very gradual. The use of RC time constant circuits at each keyer input significantly reduces the audible noise at key depression and release. However, since the keyer circuit turns on slowly, it diminishes the attack time of the keyer output signal and restricts the rapidity at which the organist can play certain musical pieces. If the organist moves too rapidly from one key to another, the keyer circuit is activated for a reduced time interval and the audio output is diminished or sluggish. In addition since there are many keyer circuits in an organ and each requires its own time constant circuit, the cost in both electrical components as well as assembly time is great.
Another common approach to eliminating audible noise due to the instantaneous DC shift in the keyer circuits is to filter the keyer output signal. Usually, the output lines of several, frequently six or more, keyer circuits which are harmonically related are tied together and applied as the input to a single filter circuit which passes the desirable audio frequency signals but attenuates the unwanted noise. This approach is restricted for use with a small number of keyer circuits because of band width limitations. Furthermore, if the cut off point of the filter is high enough to attenuate noise, the lower frequency notes sound raspy and if the cut off point is low, some percentage of the noise signal may not be attenuated and the audible thump remains in the output. In addition to the above, the component cost of the many necessary filters and the assembly time is great.
An object of the present invention is to overcome the disadvantages and deficiencies of the prior art devices.
Another object of the corrective circuit is to eliminate audible noise or thump in the audio output due to the positive and negative instantaneous DC level shift in the output waveform of the keyer circuit at key depression and release by combining with the keyer output a DC level signal of opposite polarity to the instantaneous DC shift.
Another object is to eliminate the thump in the audio output caused by each keyer circuit by using a single corrective circuit.
Other objects will be apparent from the following summary and detailed description.